﻿GEOLOGY OF THOUSAND ISLANDS REGION 



III 



thickness in each, and the two dipping toward one another, this sup- 

 position seems improbable in high degree. There seems no direct 

 evidence for it and much against it. 



The other alternative is that the structure here is anticlinal in- 

 stead of synclinal. This is a possible interpretation of it in spite 

 of the fact that the two limestones dip toward one another. Long 

 continued and severe compression may so closely compress rock 

 folds as to cause them to pass into the fan fold type as illustrated 

 in figure 8. Such folds are so pinched that vertical dips prevail 

 centrally, along the axes, and the dips farther away converge 



toward the axis in the anticlines, ^.* -^ 



instead O'f in the syn dines as in the /■" x \ 



previous case. In that also the dips / *'' %X V * 



flatten in the vicinity of the axis of ' / /---^ %1 » \ 



the fold, and pass from one direc- ', ; .'' \ / ; 



tion to the other through the hori- 

 zontal, instead of through the ver- 

 tical, as in the fan fold. In repeated 

 instances, and in many ' localities, in 

 the Grenville rocks of northern New 

 York, the writer has observed that 

 change in dip has taken place 

 through the vertical instead of 

 through the horizontal, and 

 seems to imply a condition of very close folding in the Gren- 

 ville rocks at many and widely distributed points. In this 

 especial case the dips change from the northwest to the 

 southeast through the vertical in the schists northeast of Mill- 

 site lake, but with some comparatively flat dips in the inter- 

 banded quartzites north of the lake. At the same time the schists 

 become greatly contorted and puckered. Millsite lake seems to lie 

 closely along the axis of the fold. The section shown in figure 9 

 was sketched from an exposure y 2 mile northeast of Millsite lake. 



M 



Fig. 8 Section similar to the previous, 

 surface outcrops, dips and scale the same, 

 fVifo on the assumption of fan fold structure 



Fig. 9 Exposure of Grenville rocks \ mile northeast of Millsite lake, showing sharply- 

 folded quartzite q-q, with a pinched in thin limestone between the quartzite limbs, 1, the 

 quartzite succeeded on the left by hornblende schists, s, and very schistose mica schist . 

 ms, the dip being vertical or nearly so throughout 



